It is a common practice in the oil industry the use of organic volatile solvents to improve the mobility of heavy crude, for example, light oils, condensates, kerosene, gasoils or mixtures of aromatic products. However, these solvents are not always available and, in most of cases, their use is expensive, because they are usually transported from remote locations and their use as diluents of the heavy oils reduces their economic value.
The differential trend towards the progressive reduction of actual oil reserves in some places has motivated a growing interest in the oils of lesser quality, such as heavy bitumen, oils with API densities between 8 and 22° API, oil shale and tar sands. Also, there is an increased interest in secondary and tertiary enhanced recovery methods, as well as the development of alternative energy sources.
Several research projects on enhanced oil recovery are being carried out and some of them have focused on the recovery of lesser value oils; also, other projects focused on alternative and renewable energy resources. Although the oil industry is mature, still the primary extraction methods use conventional technologies that allow a limited recovery of 15 to about 30% of the extractable reserve. Additionally, the heavy and extra-heavy crude oils present technical difficulties for handling and transportation, due to the high viscosity and low mobility. Thus, there is a need to better understand the correlation between the molecular structure of heavy crude oils and their rheological properties. Only recently the true nature of asphaltenes has been understood, which is determined by the fusion of polyaromatic hydrocarbon (PAH's) of high molecular weight. The asphaltenes are the hardest part of the heavy oils and are a class of compounds whose solubility is low or null in n-heptane, but very high in toluene. The asphaltenes are made up of molecules with 3 to 10 fused aromatic rings, which have lateral alkyl chains with about 3 to 18 carbon atoms; polar, acidic and basic groups at the edges may contain heteroatoms, like sulfur, nitrogen, oxygen and metals (Ni, V). The propensity of asphaltenes to form aggregates through π-π bonds and Van der Waals weak forces, i.e. dipole-dipole and short-range type London dispersion forces, etc. make them too difficult to handle under certain conditions, and their structural polydispersity make them complex to modeling and prediction.
API densities of typical heavy and extra-heavy crude oils are in the interval between 22-12° API for the former and 12 to 5° API for the latter.
Several studies have determined the composition of the heavy oils by separation techniques that combines analysis of major fractions of petroleum, such as simulated distillation and SARA (TLC-FID) fractionation, gas chromatography etc. Other chromatographic tests include ion exchange chromatography, exclusion size chromatography, etc., which have been applied together with the spectroscopic characterization (NMR, MS-MS, MS-FI, EPR). Recently, the use of cyclotron resonance mass spectroscopy was reported.
Among the oil recovery methods that apply in the oil industry, some use chemical injection to reduce the viscosity of heavy oils, for example, U.S. Patent Publication No. 2010/0006285 A1 discloses the use of solvents that are based mainly on petroleum fractions such as diesel and solvents (e.g. biodiesel) that are injected into the geological formation for improving fluency, e.g., Darcy's law establish the relationship v=−koΔP/μ, where v is the velocity of the fluid, ko is the permeability of the porous medium; ΔP is the pressure drop and μ is the viscosity. This expression is used to calculate the speed of the fluid, or production volume, in terms of viscosity. Also, U.S. Patent Publication No. 2004/0232051 A1 discloses a method based on ultrasound, which is applied in conjunction with an acid treatment of the crude oil, using waste oil, which reduce the viscosity approximately by a factor of 4, using a dispersed phase of acids in a continuous phase of hydrocarbons. These acids are selected from the group of mineral acids and their mixtures. Also, U.S. Pat. No. 6,279,653 discloses a method and apparatus for enhancing oil recovery, which includes the application of alkaline flows and the use of special ultrasound devices installed inside the well, thus forming an emulsion of oil and water, which is pumped to the surface more efficiently. Also, U.S. Pat. No. 3,823,776 discloses a process for enhancing the recovery of heavy oil, through the establishment of a zone of combustion in the formation, by means of the injection of a gas containing oxygen, in order to oxidize some underground oil, which promotes the formation of a combustion zone “in situ”. Subsequently, a caustic aqueous solution is injected to control the combustion, thus facilitating the production of oil. U.S. Pat. No. 7,678,745 B2 discloses the use of organic peroxides, in conjunction with an amine, which promote the reduction of oil viscosity, where the amine acts as a delaying agent. In addition, U.S. Pat. No. 5,529,930 discloses a process to reduce the viscosity of the heavy oil by means of a reaction that turns heterocyclic molecules in the oil into other molecules, whose physico-chemical properties promote a lower viscosity. Also, U.S. Patent Publication No. 2008/0257414 A1 discloses the use of an electric field during sufficient time period, to reduce the viscosity of fluids by a factor of 20%, as compared to the original fluid. U.S. Pat. No. 6,129,148 discloses a method to reduce the viscosity of oil “in situ”, by using a heat exchanger, thus reducing its viscosity to facilitate the flow into the well. Also, U.S. Pat. No. 6,544,411 B2 discloses a method to reduce the viscosity of the crude oil and its waste, approximately by a factor of 4, by ultrasonic treatment, in conjunction with the action of an organic acid or a mineral, or their combination. Also, a variety of chemical systems have been reported for reducing the viscosity of the heavy crude oils; for example, the use of primers based on free radicals (U.S. Pat. No. 4,298,455). Also, the use of polymers based on aqueous gels (U.S. Pat. No. 5,447,199) is disclosed to reduce viscosity. Additionally, some methods (U.S. Pat. No. 6,924,254) disclose treating the viscous liquids with peroxides, i.e., pentanedione and other organic peroxides (U.S. Pat. No. 6,489,282). Also, thiols and other aromatic compounds were used (EP 175511), as well as molecular recipients of free radicals (U.S. Pat. No. 3,707,459). Also, U.S. Pat. No. 6,491,053 discloses a process to reduce the viscosity of the heavy oils by mixing a liquid solvent such as kerosene, gasoline, or other aromatics and additives of low viscosity and density, for improving the pumping conditions of the heavy oils. However, some of these chemical compounds can cause a negative environmental impact, causing contamination of the wells, especially when these are high vapor pressure solvents.
In general, there are some factors that impede the recovery of the heavy oils, such as the low rock permeability, the wettability factor, the high viscosity of the heavy oils and the fluid channeling during the stimulation process, i.e., water or steam. In addition, the asphaltene precipitation under certain thermodynamic conditions may cause the obstruction of pipelines and devices in the oil facilities, thus affecting the exploitation and the operation sustainability, and causing damage to the infrastructure of the field, as well as potential production losses. In addition, due to the high sulfur content of the heavy crudes, a severe corrosion may take place in the pipelines, valves, and storage tanks. In contrast, reducing the viscosity of the heavy oil can bring technical benefits such as easing the handling along the production line, from extraction stage, transport and processing stages. Also, the final disposal of wastes should be easier and less costly, thus ensuring production, the installation maintenance and the operation sustainability.